That can be difficult, especially if the graph is plotted on axes which have been translated, rotated or have undergone some linear transformation. You have to express it in the form of a matrix, find some eigenvalues and eigenvectors and then re-express the entire matrix equation in terms of the new coordinate axes. It's difficult to explain here. What do your notes say?

On the other hand, if they are plotted on the standard coordinate axes, then use this as a rough guide: